I had the same reaction. There should be a few words showing that we now have a point which multiplied by itself (rotated by 90° around 0) goes to -1. So this number is the square root of -1.
I had the same reaction. There should be a few words showing that we now have a point which multiplied by itself (rotated by 90° around 0) goes to -1. So this number is the square root of -1.